1. Technical Field
The present invention relates to an inspection for advanced thermal reactors, fast breeders, light water reactors, plant equipment and other equipment in general, and more particularly it relates to a laser ultrasonic detection method and apparatus therefor which can be applied to in-service inspection as well as to continuous monitoring of specimens to be monitored under operation.
2. Background Art
According to prior art ultrasonic flaw inspection methods, defects such as flaws and the like are inspected by directly irradiating laser beams from ultrasonic generation laser apparatus and ultrasonic detection laser apparatus upon the surface of an inspection object of metal material or the like.
In FIG. 4 which shows an example of the prior art ultrasonic flaw detection, the system has an ultrasonic generation laser apparatus 1, an ultrasonic detection laser apparatus 2, an ultrasonic generation laser beam 3, and an ultrasonic detection laser beam 4. In FIG. 4, reference numeral 5 represents an object for inspection, 6 a flaw, and 7 an ultrasonic wave, respectively.
In FIG. 4, an ultrasonic generation laser beam 3 in a pulse form having a high energy of several tens of mJ to several hundreds of mJ is emitted from the ultrasonic generation laser apparatus 1 to irradiate and heat repeatedly the surface of the object 5 to be inspected so as to generate the ultrasonic wave 7 by causing thermal strain therein. Then, a continuous ultrasonic detection laser beam 4 with a low energy of several mJ is emitted from the ultrasonic detection laser apparatus 2 onto the surface of the object 5 to detect a minute vibration appearing on the surface of the inspection object 5, the vibration being produced when the ultrasonic wave 7 generated by the ultrasonic generation laser beam 3 reaches the surface of the object 5 after reflection from a flaw 6 or, the like.
In the prior art laser ultrasonic flaw detection methods in which both the ultrasonic generation laser beam and the ultrasonic detection laser beam are directly irradiated on the surface of an inspection object, there are the following serious problems before the methods are put into practice, thus presently impeding wide application thereof.
One of the problems is that in detection of ultrasonic signals returning from an object by the ultrasonic detection laser apparatus, when the object has irregularities or undulations on its surface as is the case with most objects, or when reflectivity of light is poor, the reflected laser beam from the object is scattered or attenuated, thereby hampering sufficient light quantities necessary for effective measurement from reaching the ultrasonic detecting laser apparatus, thus eventually rendering the ultrasonic detection thereof impossible.
Another problem is that when it is required to generate an intense ultrasonic wave in an object to be inspected by utilizing an ultrasonic generation laser beam, the power of the ultrasonic generation laser beam must also be increased. Such an increase in power can result in the surface of the object being burned.
Still another problem is that at whatever angle of irradiation an ultrasonic generation laser beam is projected onto the surface of the inspection object, most of the ultrasonic wave thus induced in the inspection object propagates in the direction perpendicular to the surface thereof, thereby allowing only a vertical flaw detection, but inhibiting the angle beam detection which is important in the ultrasonic flaw detection.
Further, there is such a problem that when there exists on the surface of the inspection object an opaque liquid, for example, molten metallic sodium or the like, which prevents transmission of the laser beam therethrough, or when the inspection object is made of a material which prevents easy generation of an ultrasonic wave with a laser beam, the ultrasonic detection cannot be applied thereto.